In recent years, EHCs (electrically heated catalysts) are attracting attention as an exhaust purification device that purifies exhaust gases discharged from engines of automobiles and the like. In EHCs, it is possible to forcibly activate a catalyst by electrical heating even under such conditions that the temperature of the exhaust gas is low and thus the catalyst cannot be easily activated, such as immediately after the engine is started, and thereby to enhance the purification efficiency of the exhaust gas.
An EHC disclosed in Patent literature 1 includes a cylindrical catalyst support having a honeycomb structure on which a catalyst such as platinum and palladium is supported, and a pair of electrodes that are electrically connected to the catalyst support and disposed opposite to each other on the outer surface of the catalyst support. In this EHC, the catalyst supported on the catalyst support is activated by electrically heating the catalyst support between the pair of electrodes. In this way, toxic substances such as unburned HC (hydrocarbon), CO (carbon monoxide), and NOx (nitrogen oxide) in an exhaust gas that passes through the catalyst support are removed by the catalytic reaction.
Since an EHC is disposed on a discharge path of an automobile or the like, the material for the above-described electrode needs to have, in addition to the electrical conductivity, heat resistance, acid resistance at a high temperature, corrosion resistance in an exhaust-gas atmosphere, and the like. Therefore, as mentioned in Patent literature 1, metallic material such as an Ni—Cr alloy and an MCrAlY alloy (M is at least one material selected from Fe, Co and Ni) is used. Meanwhile, as for the material for the above-described catalyst support, ceramic material such as SiC (silicon carbide) is used.
Since an EHC is disposed on the discharge path as described above, the above-described electrode and the catalyst support repeatedly expand and contract due to the thermal cycle (normal temperature to about 900° C.). It should be noted that there has been a problem that cracking and/or peeling occur in the electrode because of the difference between the linear expansion coefficient of the metallic material forming the electrode and that of the ceramic material forming the catalyst support. To cope with this problem, in Patent literature 2, the stress caused by the above-described linear expansion coefficient difference is alleviated by inserting a porous intermediate layer made of metallic material similar to that for the electrode between the electrode and the catalyst support.